A painting knife is a springy, shaped, metal spatula used for painting instead of a brush. The painting technique is a bit like putting butter on bread, and is especially useful for creating texture.
Painting knives comes in quite a variety of shapes and sizes. Different painting knives produce different effects, for example a short blade produces angular strokes while a long blade makes it easy to put down sweeps of color.
A painting knife is different to a palette knife, which is used for mixing paints, although many people use the same name for both.